1. If your camera has a viewfinder, make sure that the auto-switch setting between the screen and viewfinder is set to Auto or LCD Screen.
2. Gently clean the viewfinder proximity sensor if the auto-switch setting is ok. It might be dirty. Alternatively, something might be obstructing the viewfinder (a shadow or strap). Remove any foreign objects. You should still be able to switch the image output to the LCD screen manually.
3. If that doesn't work, try to reset the camera to the factory settings (initialization). Consult your product manual if you're unsure about the reset procedure.